Fluorene ring

Facial selectivities of spiro[cyclopentane-l,9 -fluorene]-2-ones 30a-30e were studied by Ohwada [96, 97]. The carbonyl tz orbital can interact with the aromatic % orbital of the fluorene in a similar manner to spiro conjugation [98-102]. The ketones 30 were reduced to alcohols by the action of sodium borohydride in methanol at -43 °C. The anti-alcohol, i.e., the syn addition product of the reducing reagent with respect to the substituent, is favored in all cases, irrespective of the substituent at C-2 or C-4 of the fluorene ring (2-nitro 30b syn anti = 68 32), 4-nitro... 

Compound 15 was dehydrofluorinated by Me3CLi in Et20 to 16, obtained as an etherate complex. After thermolysis and reduction of 16 by LiAIlLt, 17, stabilized by the aromatic fluorene rings, is generated. 

SCHEME 2.27 Synthesis of PFs with 4,5-dimethoxy substituents in the fluorene rings. (From Beaupre, S. and Leclerc, M., Macromolecules, 36, 8986, 2003.)... 

Ford et al. prepared l-methyl-9-(2-methyl-l-naphthyl)fluorene (53) in the hope that, if rotation were the true mechanism, the introduction of the 1-methyl group in the fluorene nucleus would raise the barrier by increasing the steric interaction in the transition state for rotation. They found that the barrier to rotation from the sc (S ) isomer to the ac (R ) isomer was 33.3 0.3 kcal/ mol at 166°C in hexachlorobutadiene. If this value is compared with the barrier of 9-(2-methyl-l-naphthyl)fluorene (54), 29.2 kcal/mol at 116°C, which was determined by Siddall and Stewart (93), it is clear that the introduction of the methyl group into the 1-position of the fluorene ring raises the barrier to rotation. 

Introduction of substituents into the 1-position of the fluorene ring seems to... 

The foregoing examples of differential reactivities of rotamers may be summarized by saying that the reactivity is controlled by the steric factor. The difference in the reactivities of rotamers of 9-(2-bromomethyl-6-methyl-phenyl)fluorene (56) in SN2 type reactions falls in the same category (176). However, the substituent effect is not limited to a steric one there can be conformation-dependent electronic effects of substituents as well. A pertinent example is found in the reactivity of the bromomethyl compound (56) when the rotamers are heated in a trifluoroacetic acid solution (Scheme 10). The ap form gives rise to a cyclized product, whereas the sp form remains intact (176). The former must be reacting by participation of the it system of the fluorene ring. 

A second method that has been developed more recently uses the 9-fluorenylmethyl-carboxy (Fmoc) group.148 The Fmoc group is stable to mild acid and to hydrogenation, but it is cleaved by basic reagents through deprotonation at the acidic 9-position of the fluorene ring. 

Electronic absorption studies were performed on compounds 90 and 91 in dimethylformamide (DMF) the longest wavelength maxima at 503-567 nm arising from intramolecular charge transfer (ITC) are shifted bathochromically with an increase in the sum of the nucleophilic constant btt-attached to the fluorene ring. An equation to enable calculation of this effect was devised <2001MM2232>. 

With the monocationic species, no fluorene cyclization is observed. However upon addition of CF3SO3H, the cyclization occurs almost quantitatively. This is consistent with formation of the protonated, dicationic intermediate (46) leading to the cyclization product (59). In this same study, it is noted that other stable monocationic 1,1-diarylethyl cations (i.e., the 1,1-diphenylethyl cation) do not readily form the fluorene ring system, indicating the importance of superelectrophilic activation. 

The photoconductive properties of the fluorene-derived polymers 19, 23, and 24 were studied by the single-layer photocells in the configuration of ITO/Pt polyyne/Al. These polymers show moderate photoconductivity. A PCQE of 0.01 % was estimated in these cases, which does not vary much with variation of the central fluorene ring.26,30 Likewise, polymer 44 was also shown to be a photoconducting material in a similar architecture with a PCQE of 0.01 % in the forward bias mode. 

The protecting group is removed using a base. This works because of the acidity of the proton in position 9 on the fluorene ring. Removal of that proton causes a breakup of the molecule with the release of the amine at the end. 

The Step 2 product (2.98 mg) and 1,3-dinitrobenzene (1.80 mg) were dissolved in 10 ml of THF and then diluted 100 times. This stock solution was used for absorption spectrum measurements in a 10 mm quartz cell at ambient temperature. The intensity was 0.140 at 242 nm, which changed to 0.108 after the addition of 1,3-dinitrobenzene. By changing the 1,3-dinitrobenzene concentration from 2.5 x 10 Mand5.0x 10 M at 242 nm, the absorption intensity changed from to 0.077 and 0.059, respectively. This hypochromic effect demonstrated that the fluorene ring of the polymer and 1,3-dinitrobenzene formed a stacked complex. 

Positional Selectivity in FN. The reactions of nucleophiles with FN (for structures, see the introduction) present an interesting problem. In water, the nucleophile interacts with position 9 of the fluorene ring, whereas in dipolar aprotic solvents such as (CH3)2SO, the nucleophile attacks C-a ... 

No chemical reactions have been observed which require the intermediacy of the B2-type cation radical. However, CIDNP experiments with 33 provide evidence for the existence of the unique structure of 33 in which two cyclopropane bonds are simultaneously weakened. It is noteworthy that spiroannelation with the fluorene ring causes a remarkable change in the distribution of the charge and odd electron densities in the ring of cyclopropane cation radicals. 

In connection with the spiroannelation phenomenon, the final example in this section focuses on the reactivities of bicyclo[6.1.0]nona-triene cation radicals. Spiroannelation at C-9 with a fluorene ring dramatically changes the rearrangement pathway followed in their SET reactions. CA-sensitized photoreaction of 34a gives 1,3,5,7-cy-clononatetraene (38) whereas the DCA-sensitized photoreaction of the diphenyl derivative 34b affords the cycloheptatriene derivative 39 as the sole product. Remarkably, the DCA-sensitized photoreaction of 34c gives the barbaralane derivative 40 as a major product. 

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